Secondary batteries that output high voltage, which are represented by lithium ion secondary batteries, include a non-aqueous electrolyte. The non-aqueous electrolyte includes a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. As the solute, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) etc. are used.
The non-aqueous solvent includes a chain carbonate having low polarity but low viscosity, a cyclic carbonate having high polarity but relatively high viscosity, a cyclic carboxylic acid ester, a chain ether, cyclic ether etc. Examples of the chain carbonate include diethyl carbonate (DEC). Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC).
Various studies have been made on composition of the non-aqueous electrolyte.
Patent Literature 1 proposes a non-aqueous electrolyte including EC, PC, and DEC in a volume ratio of 1:2:7. When DEC having low viscosity is used as the main component, favorable low-temperature characteristics can be obtained.
Patent Literature 2 proposes a non-aqueous electrolyte including a fluorine-containing aromatic compound. By adding a fluorine-containing aromatic compound, deterioration of rate characteristics accompanying charge and discharge cycles can be suppressed.